1. Field of the Invention
This invention relates to processes for the oxidation of materials. In another aspect, this invention relates to processes for the oxidation of materials in the presence of water at supercritical water temperatures and optionally at supercritical water pressures. In still another aspect, this invention relates to oxidation of materials in the presence of water at supercritical water temperatures and optionally at supercritical water pressures, wherein the initial oxidation rate is enhanced by the presence of small amounts of strong oxidizing agents.
2. Description of the Related Art
The use of oxidation reactions to destroy organic molecules by combining them with oxygen to form carbon dioxide, water and inorganic compounds is the basis of many waste treatment methods ranging from the biological treatment used for sewage to the high temperature incineration used for hazardous waste. The required destruction efficiency and the relative costs and throughput capabilities of candidate methods affect the choice of destruction method for a particular waste. For hazardous wastes, the public generally requires that destruction efficiencies approach one hundred percent.
Incineration is capable of achieving the high destruction efficiency required for hazardous waste destruction. The high temperature oxidation reactions which take place in incinerators are rapid free radical reactions. Innovations such as the fluidized bed and the rotary kiln have improved the mass transport and mixing of oxidant and waste to optimize destruction efficiency. Incinerators, however, have met with public resistance and permitting difficulties largely because of the gaseous emissions which may contain unreacted toxic compounds, products of incomplete combustion or acid rain precursors. Incineration is also limited to solid wastes, organic liquids, or relatively concentrated aqueous solutions of organic compounds; otherwise prohibitive amounts of auxiliary fuel must be added to provide sufficient heating value to maintain optimum incineration temperature.
Wet Air Oxidation (or wet oxidation) is a process that overcomes some of the disadvantages of incineration. Wet Air Oxidation is a two phase oxidation in which the liquid phase contains that material to be oxidized and the vapor phase contains the oxidant, and the reaction takes place essentially in the liquid phase. This technology has been around since the early 1960's. Conventional Wet Air Oxidation is carried out at relatively low, subcritical temperatures (150.degree.-370.degree. C.) and elevated pressures (30-250 bar). Wet Air Oxidation provides oxidation efficiencies in the 50 to 85 percent range, which is not an adequate destruction efficiency for many hazardous wastes, but it is useful in reducing the toxic components of the waste to smaller molecular weight organic acids and aldehydes. These low molecular weight, oxygenated compounds resist further chemical oxidation but can be treated biologically or with activated carbon.
The mechanism of the wet oxidation reaction is not well understood. See, Rulkens et al., "Feasibility Study of Wet Oxidation Processes For Treatment of Six Selected Waste Streams", Report DBW/RIZA nota 89-079, Ministry of Transport and Public Works, The Netherlands (1989). While some data seems to argue against a free radical mechanism, see, Weygandt, "High Pressure Oxidation of Organic Compounds in Aqueous Solution", University Microfilms, Inc. (1969), other researchers have found an induction period for the oxidation reaction, which can be described as the time needed to obtain a critical concentration of free radicals, followed by a steady-state period, which suggests that the reaction does proceed by a free radical mechanism. See, Willms et al., "Aqueous-Phase Oxidation: The Intrinsic Kinetics of Single Organic Compounds", Ind. Eng. Chem. Res., vol. 26 at 148 (1987). See also, Skaates et al., "Wet Oxidation of Glucose", Can. J. of Chem. Eng., vol. 59 at 517 (1981). At low temperatures, however, the reaction rate is slow and wet oxidation requires long residence times. While increasing the temperature increases the reaction rate and the organic carbon degradation, it also increases the corrosion rate and the energy requirements for the process. Since it is desirable to operate a wet oxidation reactor at low temperature, the use of catalysts to increase the reaction rate at low temperatures has been explored.
The catalytic effect of transition metal cations, particularly Cu(II) and Fe(III), has been demonstrated for wet oxidation of wastes. See, Imamura et al., "Wet-Oxidation of Acetic Acid Catalyzed by Copper Salts", J. Japan. Petrol. Inst., vol. 25(2) at 74 (1982), and see, Randall et al., Detoxification of Specific Organic Substances by Wet Oxidation", J. Water Pol. Cont. Fed., vol. 52(8) at 2117 (1980). U.S. Pat. No. 3,912,626, issued Oct. 14, 1975 to Ely et al., discloses the use of Cu(II) and Ag(I) cations to improve the degradation of wastewater chemical oxygen demand by wet oxidation. Other studies have shown that the addition of hydrogen peroxide (H.sub.2 O.sub.2), along with various cations enhances the catalytic effect by providing a source of free radicals. See, Bishop et al., "Hydrogen Peroxide Catalytic Oxidation of Refractory Organics in Municipal Wastes", I&EC Process Design and Dev, vol. 7 at 110 (1968), and see, Chowdhury et al., "Catalytic Wet Oxidation of Strong Waster Waters", AICHE Symposium Ser., vol. 71, no. 151 at 46 (1975).
In U.S. Pat. No. 3,984,311, issued Oct. 5, 1976 to Diesen et al , it is disclosed that a mixture of nitrate and iodide or bromide ions increases the rate of the wet oxidation reaction. Miller, in U.S. Pat. No. 4,212,735, issued Jul. 15, 1980, teaches that the addition of a transition metal ion to the nitrate/iodide(bromide) system enhances the rate increase effect. The nitrate/iodide(bromide) system, however, undergoes oxidation/reduction reactions in the process and is, therefore, a reactant and not strictly speaking a catalyst. Similarly, dichromate in combination with hydrogen peroxide also increases the rate of wet oxidation, but the Cr(VI) in the dichromate ion is reduced to Cr(III). See, Brett et al., "Wet Air Oxidation of Glucose with Hydrogen Peroxide and Metal Salts", J. Apol. Chem. Biotechnol., vol. 23 at 239 (1973). Both nitrate and dichromate may be classified as auxiliary oxidizing agents rather than as catalysts.
U.S. Pat. No. 4,460,628 issued Jul. 17, 1984 to Wheaton et al., discloses that heterogenous catalysts consisting of either elemental metals or solid metal oxides or salts provide some catalytic effect in wet oxidation. See also, Imamura et al. "Wet Oxidation Catalyzed by Ruthenium Supported on Cerium (IV) Oxides", Ind. Eng. Chem. Res., vol. 27 at 718 (1988), and see, Chowdhury et al., supra.
The processes disclosed in U.S. Pat. No. 4,294,706 issued 1981 to Kakihara et al. and in U.S. Pat. No. 4,141,828, issued Feb. 27, 1979 to Okada et al., depend on the use of a heterogeneous catalyst for conversion of ammonia to nitrogen at the low temperatures used in wet oxidation.
U.S. Pat. No. 4,751,005 issued Jun. 14, 1988 to Mitsui et al. teaches the use of a mixed metal/metal oxide catalyst for wet oxidation of waste water, and further teaches the use of molecular oxygen and ozone and/or hydrogen peroxide as an oxidizing agent for wet oxidation of waste water.
While Wet Air Oxidation is in some aspects an improvement over incineration in the disposal of hazardous wastes, it still does not achieve a high enough oxidation of organic carbon to CO.sub.2. In addition, wet oxidation residence times are relatively high which increases the reactor cost.
Wet oxidation at supercritical temperature and optionally supercritical pressure has been suggested as an improvement over conventional wet oxidation.
Supercritical Water Oxidation, described in U.S. Pat. Nos. 4,338,199, issued Jul. 6, 1982, and 4,543,190, issued Sep. 24, 1985, both to Modell and U.S. Pat. No. 4,822,497, issued Apr. 18, 1989 to Hong et al., all hereby incorporated by reference, can achieve the high destruction efficiency required for hazardous waste destruction and it is applicable to dilute aqueous waste streams containing less than about 20 percent organic carbon which cannot be economically incinerated. The Supercritical Water Oxidation reaction takes place at elevated supercritical water temperatures (374.degree.-700.degree. C.) and supercritical water pressures (greater than about 221 bar) in a homogeneous supercritical fluid.
A counterpart to Supercritical Water Oxidation is Semicritical Water Oxidation, described in more detail below, which takes place at elevated supercritical temperature (374.degree.-700.degree. C.), but at subcritical pressures between 25 bar and the critical pressure of water.
Although the supercritical temperature range utilized in both Supercritical and Semicritical Water Oxidation is significantly lower than those typical of incineration, the oxidation reactions in both Supercritical and Semicritical Water Oxidation are also rapid free radical reactions. Therefore, residence time under one minute is sufficient for complete conversion of most organic compounds to carbon dioxide, water, and inorganic compounds at around 550.degree. C.
In Supercritical and Semicritical Water Oxidation of wastes, the pressurized aqueous waste stream or waste mixed with water is fed into a tubular or vessel reactor along with compressed air or oxygen as the oxidant. The processes may also be carried out below ground in a deep well reactor as disclosed in U.S. Pat. No. 4,792,408, issued Dec. 20, 1988 to Titmas. U.S. Pat. No. 4,861,497 to Welch et al. teaches the replacement of air or oxygen as the oxidizing agent in Supercritical Water Oxidation with liquid phase H.sub.2 O.sub.2, ozone, inorganic oxides that decompose to yield oxygen, or mixtures of the above, within a combination heat exchanger/reactor. The possibility of using a sodium hypochlorite solution as the oxidizing agent has been suggested. See, Snow and Levi, "Testing of Supercritical Water Oxidation (SCWO) at CAMDS", Technical Report 03-01, Tooele Army Depot, Tooele, Utah, October, 1990.
In a report coauthored by E. F. Gloyna, it was suggested that a mixture of oxygen and hydrogen peroxide could be used as the oxidizing agent, although no mixture proportions or experimental data have been given. See, Shanableh and Gloyna, "Subcritical and Supercritical Water Oxidation of Industrial, Excess Activated Sludge", Technical Report CRWR211, Center for Research in Water Resources, U. of Texas, Austin, Tex., at page 79 (1990). However, in a subsequent report out of the University of Texas, also coauthored by Gloyna, it is disclosed that the initial reaction rates for subcritical water oxidation using oxygen are expected to be lower as compared to hydrogen peroxide due to the limited mass-transfer rate between the gas and liquid phases of the subcritical water oxidation reaction. It is further disclosed that for supercritical water oxidation (a single phase reaction) the effectiveness of oxygen and hydrogen peroxide as oxidants is kinetically comparable. See, Li, Chen and Gloyna, "Kinetic Model for Wet Oxidation of Organic Compounds in Subcritical and Supercritical Water", Annual Meeting, Nov. 17-22, 1991, at pages 4-5.
U.S. Pat. No. 5,075,017, issued Dec. 24, 1991 to Hossain et al. discloses that mixtures of oxidants can be used in supercritical water oxidation. However, no proportions or data are presented.
While liquid phase oxidants may be useful in certain situations, they are not in general preferred due to their higher cost. For example, H.sub.2 O.sub.2 costs about forty times more than pure oxygen per unit of available oxygen, while nitric acid costs three times more.
As the waste/oxidant mixture is rapidly heated above the critical point of water (374.degree. C.) in Super- or Semicritical Water Oxidation, a rapid oxidation reaction converts the organic carbon and hydrogen to CO.sub.2 and H.sub.2 O. Inorganic salts and oxides are typically relatively insoluble in the fluid/gas phase and precipitate in the reactor. Inorganic acids such as the haloacids, HX (X=F, Cl, Br or I), sulfuric acid, H.sub.2 SO.sub.4, or phosphoric acid, H.sub.3 PO.sub.4, are formed when heteroatomic molecules containing halogens, sulfur or phosphorus are oxidized. These acids remain in the gas phase unless they are neutralized in situ by addition of a caustic material such as NaOH to the reactor. Neutralization produces salts which, as mentioned, precipitate in the reactor.
When a vessel reactor is used, any precipitates deposited in it can be removed from the bottom in an aqueous brine solution or slurry as disclosed in the Hong '497 patent. The brine phase is formed by creating a temperature gradient down the length of the reactor vessel so that the bottom of the vessel is below the critical temperature of water and an aqueous phase forms.
Although the Super- or Semicritical Water Oxidation reaction is rapid, some refractory compounds react more slowly and require higher temperature or longer residence time for complete conversion. Modell (U.S. Pat. No. 4,338,199) suggested the use of common metal oxide or supported metal catalysts in the Supercritical Water Oxidation process. Webley, "Fundamental Oxidation Kinetics of Simple Compounds in Supercritical Water", Ph.D. Dissertation, Massachusetts Institute of Technology, 1989, has suggested the addition of free radical sources and initiators such as H.sub.2 O.sub.2 to Supercritical Water Oxidation systems. The catalytic effect of nickel on the oxidation of carbon monoxide and ammonia has been investigated with mixed results. See, Helling et al., "Oxidation Kinetics of Carbon Monoxide in Supercritical Water", J. of Energy and Fuels, 1987, volume 1 at 417, and see, Webley et al., "Oxidation Kinetics of Ammonia and Ammonia-Methanol Mixtures in Supercritical Water in the Temperature Range 530.degree.-700.degree. C. at 246 Bar", Ind. Eng. Chem. Res., Volume 30. at 1745, 1991. While nickel appeared to have no significant catalytic effect on carbon monoxide oxidation, it did increase the rate of ammonia oxidation. The enhancement of the ammonia oxidation rate, however, was less than would be expected for a purely catalytic reaction which suggests a more complex mechanism. Others have postulated that inorganic components of the waste feed to the process may provide a catalytic effect, but this has not been studied.
A variation of the Supercritical Water Oxidation process, is the hybrid wet oxidation/Supercritical Water Oxidation process which is described in U.S. Pat. No. 4,292,953, issued Oct. 6, 1981 to Dickinson, hereby incorporated by reference. In Dickinson's scheme, the oxidation reaction is initiated under subcritical wet oxidation conditions. As the reaction proceeds heat is released and the process stream temperature eventually exceeds the critical temperature of water. The heatup to the critical temperature may be aided by the addition of external heat, with for example, an electrical or fired heater, or regenerative heat exchange. As described, this process typically uses an alkali catalyst such as sodium carbonate or calcium carbonate. Another variation would be a hybrid wet oxidation/Semicritical Water Oxidation process.
Supercritical Water Oxidation, Semicritical Water Oxidation and the hybrid oxidation schemes, all have an advantage over wet oxidation and incineration for waste destruction in that all provide complete oxidation of organic carbon to CO.sub.2 with no noxious by-products in relatively short residence times. However, while the residence times in these methods are relatively short, it is still desirable to further reduce the residence time. The initial reaction rate is an important parameter in reactor design. A faster initial reaction rate would mean a reduction in the required residence time to achieve a given destruction efficiency. A shorter required residence time can minimize the efflux of partially oxidized by-products and optimize plant throughput capability. It can facilitate complete conversion of refractory compounds such as acetic acid to CO.sub.2 at moderate temperatures.
Therefore, a need exists in the oxidation art for a way to increase the oxidation reaction rate so that residence times can be reduced thereby minimizing the efflux of partially oxidized by-products and optimizing plant throughput capability.